Ultra wideband-type technology is distinguished from narrowband and spread spectrum technologies in the sense that the bandwidth of the signal of ultra wideband type is typically between about 25% and about 100% of the central frequency. Moreover, instead of transmitting a continuous carrier modulated with information or with information combined with a spreading code, which determines the bandwidth of the signal, ultra wideband technology involves the transmission of a series of very narrow pulses. For example, these pulses may take the form of a single cycle, or monocycle, having a pulse width of less than 1 ns. These pulses that are extremely short in the time domain, when transformed into the frequency domain produce the ultra wideband spectrum that is characteristic of UWB technology.
In UWS technology, the information transmitted by the signal can be encoded, for example, by a modulation technique called “pulse position modulation” (PPM). In other words, the information encoding is carried out by varying the instant of transmission of individual pulses. More specifically, the pulse train is transmitted at a frequency of repetition that can be as much as several tens of MHz. Each pulse is transmitted in a window of predetermined length, for example, 50 na. Compared to a theoretical position of transmission, the pulse is then ahead or delayed, enabling a “0” or a “1” to be encoded. More than two values can also be encoded by using more than two positions shifted relative to the reference position. It is also possible to superimpose a BPSK modulation on this position modulation.
In view of the central frequency of the pulses, which is generally of the order of a few GHz, and the positional shift of the pulses with respect to the theoretical position, which is for example of the order of a few tens of picoseconds, it then becomes necessary to use clock signals having very high frequencies, for example of the order of about 100 GHz. Now, this necessitates the use of means which are constraining both from the technological point of view and from the point of view of the consumption of current. Thus, conventionally, this has not been implemented in CMOS technology.
Moreover, it is important that the precision of the clock also be very good, typically of a few picoseconds, thereby adding further technological constraints.